I think it depends on the context, the situation and the nuance of the speaker.
He will do his work.
彼は(彼の)仕事をします。(simply talk about the future)
彼は(彼の)仕事をしますよ (仕事をするはずです)。(near to "be supposed to / be sure to", with きっと or ぜったい, the possibility is very near to 100%)
I will come and meet you, but first I want to rest.
私はあなたにあいにきます。(simply talk about the future)
私はあなたにあいに来るでしょう。(the speaker isn't very sire if "I" will / will be able to come)
私はあなたにあいにきますよ。(the speaker really has the "will" to come and meet you, with ぜったい, the speaker is 100% sure)
I hope this was helpful.